Messages in this thread | | | Date | Tue, 19 Sep 2006 13:40:15 -0400 (EDT) | From | Alan Stern <> | Subject | Re: Uses for memory barriers |
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On Wed, 20 Sep 2006, Nick Piggin wrote:
> >>(P1): If each CPU performs a series of stores to a single shared variable, > >> then the series of values obtained by the a given CPUs stores and > >> loads must be consistent with that obtained by each of the other > >> CPUs. It may or may not be possible to deduce a single global > >> order from the full set of such series. > > > > > > Suppose three CPUs respectively write the values 1, 2, and 3 to a single > > variable. Are you saying that some CPU A might see the values 1,2 (in > > that order), CPU B might see 2,3 (in that order), and CPU C might see 3,1 > > (in that order)? Each CPU's view would be consistent with each of the > > others but there would not be any global order. > > > > Somehow I don't think that's what you intended. In general the actual > > situation is much messier, with some writes masking others for some CPUs > > in such a way that whenever two CPUs both see the same two writes, they > > see them in the same order. Is that all you meant to say? > > I don't think that need be the case if one of the CPUs that has written > the variable forwards the store to a subsequent load before it reaches > the cache coherency (I could be wrong here). So if that is the case, then > your above example would be correct.
I don't understand your comment. Are you saying it's possible for two CPUs to observe the same two writes and see them occurring in opposite orders?
> But if I'm wrong there, I think Paul's statement holds even if all > stores to a single cacheline are always instantly coherent (and thus do > have some global ordering). Consider a variation on your example where > one CPU loads 1,2 and another loads 1,3. What's the order?
Again I don't follow. If one CPU sees 1,2 and another sees 1,3 then there are two possible global orderings: 1,2,3 and 1,3,2. Both are consistent with what each CPU sees. If a third CPU sees 2,3 then the only consistent ordering is 1,2,3.
But in the example I gave there are no global orderings consistent with all the observations. Nevertheless, my example is isn't ruled out by what Paul wrote. So could my example arise on a real system?
Alan
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